Tuesday, August 12, 2014

Beware of the hills.


Frequency of landslides in India is indeed increasing. 



By
Dr. Nitish Priyadarshi
Geologist.

Malin, the village in Pune district that was flattened by a landslide few days back claiming more than 130 lives, has brought the focus back on the management –or mismanagement – of the vulnerable hills of India especially Western Ghats, Himalayas and North Eastern states.

The torrential rain on July 30 near Pune perhaps would not have brought down a side of the hill had it not been weakened by quarrying, leveling and deforestation activities banned in the old weathered hills and ecologically sensitive areas.

Last year it was Uttrakhand this year it is Malin and Nepal. A massive landslide in Nepal  triggered by continuous monsoon rains, caused 10 deaths, displaced 5,000 families and destroyed dozens of houses. The landslide has also blocked the Sunkoshi River and forming a lake which is threatening to cause downstream flash floods.

Heavy rainfall in June last year wreaked havoc across Uttarakhand, causing rivers and glacial lakes to overflow and triggering massive landslides – killing almost 6,000 people. Construction of hydroelectric dams, deforestation and the spread of unregulated buildings along riverbanks magnify the impact of the monsoons.

Development works carried out in pursuit of greater economic growth – such as the construction of dams and deforestation – are putting people and the environment at greater risk when disasters strike.
India is one of the most disaster-prone countries in the world, and many of its 1.2 billion people live in areas vulnerable to natural hazards such as floods, landslides, cyclones, droughts and earthquakes.


Landslides are a natural hazard that affect at least 15 percent of the land area of our country, covering an area of more than 0.49 million sq. km. landslides of different types occur frequently in the geo-dynamically active domains in the Himalayan and North-Eastern parts of the country as well as relatively stable domain in the Western Ghats and Nilgiri hills in the southern part of the country. Besides, sporadic occurrences of landslides have been reported in the Eastern Ghats, Ranchi plateau, and Vindhyan plateau, as well. In all 22 states and parts of the Union Territory of Pudducherry and Andaman and Nicobar Islands of our country are affected by this hazard, mostly during monsoons.

Landslides Zonation Mopping is a modern method to identify landslides prone areas and has been in use in India since 1980s.The major parameters that call for evaluation are as follows:
  1. Slope-Magnitude, length and Direction
  2. Soil thickness
  3. Relative relief
  4. Land use
  5. Drainage- pattern and density
  6. Landslide affected population
Causes of Landslides

Landslides can be caused by
1. Poor ground conditions
2. Geomorphic phenomena.
3. Natural physical forces
4. Quite often due to heavy spells of rainfall coupled with impeded drainage.
A Checklist of Causes of Landslides Ground Causes
  1. Weak, sensitivity, or weathered materials
  2. Adverse ground structure (joints, fissures etc.)
  3. Physical property variation (permeability, plasticity etc)

The Himalayan mountain ranges and hilly tracts of the North-Eastern region are highly susceptible to slope instability due to the immature and rugged topography, fragile rock conditions, high seismicity resulting from proximity to the plate margins and high rainfall. Extensive anthropogenic interference, as part of developmental activities, is another significant factor that increases this hazard manifold. As a result, the landscape in the Himalayan and North-Eastern regions is highly susceptible to reoccurrence of landslides.

Similarly, the Western Ghats overlooking the Konkan coast, though located in a relatively stable domain, experience the fury of this natural hazard due to steep hill slopes, overburden and high intensity rainfall. The Nilgiri hills located at the convergence zone of the Eastern Ghats and the Western Ghats bear the innumerable scars of landslides due to their location in a zone of high intensity and protracted rainfall where overburden is sensitive to over-saturation.

Vast areas of western Sikkim, Kumaon, Garhwal, Himachal Pradesh, Kashmir, and several other hilly regions have been denuded of protective vegetal cover, which has been reduced to less than 30 percent, which is less than half of what would be considered desirable. As the pressure of population grew rapidly, more and more human settlements, roads, dams, tunnels, water reservoirs, towers and other public utilities came up in vulnerable areas. The road network in the Himalayan region is more than 50,000 km in length. A large number of dams have been built in the Himalayan region. Quarrying and mining, for example, in the Doon valley, Jhiroli (Almora) and Chandhak (Pitthoragarh) have inflicted heavy damages to the slopes and the associated environment.

According to the information obtained under RTI, In Mumbai city over 22,483 hutment in 327 hilly areas across 25 Assembly constituencies in the city, including Western and Eastern suburbs, are dangerous and the people living there need to be shifted as soon as possible. In the main city, 49 spots are dangerous in which total hutments are 3986, while in Mumbai Suburb 278 spots are most dangerous.

An overall evaluation of the pattern and nature of landslide occurrences in the Kerala part of Western Ghats and its corresponding eastern flank falling within Tamil Nadu reveals the following main features:

1.      Almost all mass movements occur during monsoons (SW and NE monsoon) in the western flank of Western Ghats and during occasional cyclonic events in the eastern flank indicating that main triggering mechanism is the over- saturation of overburden caused by heavy rains.
2.      There seems to be a relation between intensity of rainfall and slope failures.
3.      Majority of the catastrophic mass movements is confined to the overburden without affecting the underlying bedrock.
4.      Improper land use practices such as heavy tilling, agricultural practices and settlement patterns have contributed to creep and withdrawal of toe support in many cases.
5.      A common factor noticed in most of these vulnerable slopes deforestation in the recent past, cultivation of seasonal crops and increase in settlements.
6.      In some areas developmental activities like construction of buildings, road cutting, embankments, cut and fill structures causes modification of natural slopes, blocking of surface drainage, loading of critical slopes and withdrawal to toe support promoting vulnerability of critical slopes.

Frequency of landslides is indeed increasing. The main reasons for this are primarily unplanned development in landslide prone areas which can change the geomorphology of that area. Because of accelerated deforestation, rampant urbanization, high frequency of earthquakes, fragile geological structures, steep topography and intense rainfall in the mountainous regions of the South Asia, the number of fatal landslides, casualties and economic loss is increasing year by year. Increasing extreme events due to climate change are also responsible for this phenomenon. Rainfall is becoming extreme in India, either heavy or scanty. If there is a lot of rainfall in a short span of time, then the soil is not able to absorb the moisture, which makes it vulnerable to erosion and slope instability, eventually leading to landslides.



The Himalayan and North-Eastern regions are potential sites where landslides dams have formed at many places in the past and the potential of such occurrences in the future is high.

Reference:

National disaster management guidelines, June 2009. National disaster management authority, Government of India.

 
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